The efficacy of allergen immunotherapy (IT) in the treatment of asthma in adults remains a matter of controversy due to limited and inadequate prior studies. This prospective double-blind placebo controlled study is designed to rigorously test the hypothesis that mite immunotherapy causes immunologic and clinical improvement in a population of mite sensitive adult asthmatics. Mite sensitive, adult asthmatics with no evidence of significant non-asthmatic cardiopulmonary disease who require regular non-steroid asthma medications will enter a 6-month baseline period. At the end of the baseline period, they will be randomized to a mite or placebo immunotherapy group with stratification on the basis of high or low mite bronchial sensitivity and high or low mite antigen exposure. Throughout the baseline and treatment period, subjects will keep daily symptom, medication, and peak expiratory flow rate diaries; they will be clinically evaluated monthly; and home mite environmental assessments will be obtained every 3 months. During the baseline period, at maintenance IT and at 6 months, 1, 2, and 3 years after maintenance IT, dilutional mite skin testing, serum mite-specific IgE and IgG, methacholine sensitivity, and mite antigen bronchial challenge with mediator assessment will be performed. At the end of the study, the clinical and immunologic parameters described above will be compared in mite and placebo groups at the time intervals described above. In addition, using co-variate analysis, clinical or immunologic parameters which correlate with a clinical response to IT will be identified.